We determined the activity of tetroxoprim (TXP) and sulphadiazine (SDZ) alone against a variety of bacterial species comprising 199 clinical isolates, using an agar dilution technique. Compared to trimethoprim (TMP), TXP was in general 8-16 times less active although there was variation between bacterial species. There was complete cross-resistance between the two compounds both qualitatively and quantitatively. SDZ had the same or slightly lesser activity than sulphamethoxazole (SMZ). Again there was complete cross-resistance between SDZ and SMZ. In combination TXP/SDZ showed synergistic activity against 21 strains of enterobacteria and Staph, aureus, unless high level resistance to sulphonamide was present. There was variable synergy with N. gonorrhoeae and none with Strep, faecalis and Ps. aeruginosa. Combined activity was also investigated using H. influenzae (7 strains) and Strep, pneumonia (12 strains), against which TXP/SDZ showed invariable synergy, often to a considerable degree. In an epidemiological survey of 2000 urinary isolates only 24 (1 2%) showed resistance to TMP. All these strains showed resistance to TXP as well. The remaining strains showed sensitivity to TXP and TMP. © 1979 Oxford University Press.
